1. Field of the Invention
The invention relates to a process for preparing organopolysiloxane compositions in a kneading cascade having at least two kneading chambers which are arranged in series next to one another and a continuously operating double-trough kneader.
2. Background Art
EP 570387 A describes a process for the continuous preparation of HTV silicone compositions, in which the incorporation of filler into the polysiloxane and the degassing are carried out in one step in a single-screw reciprocating kneader. However, the machine can be separated so that the mixing process can take place in the first kneader and the degassing step can take place in the second kneader. Owing to phase inversion, dry friction and the mechanical destruction of high molecular weight polyorganosiloxanes, the storage stability of the product is reduced. In addition, satisfactory throughputs cannot be achieved using this machine concept.
A further process development is described in EP 1033389A. To improve the storage stability in particular, the process is extended by a rubber recirculation which is supposed to prevent phase inversion and thus is supposed to reduce the tendency of the HTV compositions prepared in this way to become stiff and brittle. Two-stage processes are not mentioned in EP 1033389A. The process is made very complicated in engineering terms by the recirculation of product, so that economical operation is not possible. The throughout is also unsatisfactory in this process since the recirculation of product greatly reduces the space-time yield of the plant.
EP 1110696 A describes a two-stage process for preparing HTV silicone compositions. In the first part of this process, mixing of filler and silicone polymer and also hydrophobicizing agents and plasticizers takes place in a corotating twin-screw extruder. In the second part of the process, the mixture is then compounded fully and degassed in a single-screw reciprocating kneader. Compared to EP 570387 A, the process of EP 1110696 A is supposed to be superior in terms of throughput and improved filler distribution. However, EP 570387 A states that twin-screw machines are problematic in terms of machine wear and abrasion of metal and can result in undesirable gray coloration of the HTV compositions produced. This applies particularly to highly filled stiff mixtures.
EP 807509 A describes the continuous preparation of storage-stable organopolysiloxane compositions using a specific cascade of kneading machines. The objective is to produce base compositions for addition-crosslinking liquid rubbers. Degassing of the composition occurs only via a degassing vessel, not by means of a specifically designed machine. According to EP 807509 A, HTV compositions can also be prepared using a kneading cascade. Owing to the lack of opportunities for degassing or unsuitable degassing facilities, HTV rubbers produced by this process are unusable, however.
EP 1468803 A describes a process in which a kneading cascade comprising at least two kneading chambers which are arranged in series next to one another and each contain two kneading tools which have parallel axes and are capable of being driven in corotating or counterrotating directions and which are connected to one another by means of openings through which material can pass in a direction transverse to the axes of the kneading tools is used. From this kneading cascade, the now plastic crude rubber is pumped into a reciprocating kneader in which the material can be kneaded and degassed.
This process has the disadvantage that technical limits are reached which do not permit throughputs averaged over all products of greater than 1500 kg/h are reached. Various factors are limiting. Limiting factors are firstly the introduction of filler and secondly the residence time since the average residence time inevitably decreases when the throughput is increased, which has an adverse effect on quality. The adverse effect becomes particularly clear in the properties of: yellow coloration in the case of peroxidic crosslinking; bubble formation during extrusion; and storage stability. The process described in EP 1468803 A does not contain any buffer facilities, so that in the event of a malfunction backing up of material immediately occurs and the plant has to be shut down or crude rubber has to be discharged and then processed further in another plant.